If you are headed to that gracious, Southern city on the Ohio River, don't forget your passport. The recently expanded Urban Bourbon Trail has 27 bars and restaurants, mostly downtown, but trailblazers can "pick six," getting a passport stamp at each establishment to become Official Bourbon Country Citizens.

Citizenship entitles imbibers to many rights and privileges. OK, not really. They just get a T-shirt and a certificate, but after reaching a state of bourbon-induced bliss, nobody cares.

So, what exactly is bourbon? Here's a little tutorial for the uninitiated.

All bourbon is whiskey, but not all whiskey is bourbon. Bourbon is American-made whiskey that must be distilled from a mash that is 51 percent corn and aged in charred, new white oak barrels for a minimum of two years, imbuing it with that lovely golden-brown color. Other whiskey can age in re-used barrels.

The law prohibits bourbon distilleries from adding anything except water to their products, but other whiskey makers can add artificial colors and flavors.

Establishments on the Urban Bourbon Trail each stock a minimum of 50 bourbons, but some have closer to 150. So even sippers with a passion for rare or expensive bourbons — like the elusive Pappy Van Winkle, aged for 15, 20 or 23 years — are likely to find it on hand.

Most of the world's bourbon is made in tiny Bardstown, just 35 miles south of Louisville. Plenty of it makes its way to the city's swankiest bars and cocktail lounges, so put on your fancy pants and let's go.

It's impossible to visit them all, but here are a few of the most celebrated establishments, ones that offer real insight into Kentucky's bourbon heritage.

If you're serious about getting six stamps on your passport, don't start at Buck's Restaurant and Bar in the Old Louisville historic district. Once you settled in at the bar, it's hard to leave.

You may think you will only have one drink, perhaps a shot of your favorite single-barrel bourbon, but like so many before you, you will be seduced by the ubiquitous white bouquets of flowers and the soft jazz music trilling from the piano. When the bartender asks if you will have another, you will.

After a while, you will inquire about that delightful aroma wafting from the kitchen, and before you know it, you are being escorted by a smiling hostess from bar to table, something you never intended. Soon, you are feasting on delicious country-fried quail stuffed with sausage dressing and bathed in bourbon peppercorn gravy.

Something far in the back of your mind nudges you to move along to your next stop, but when the server suggests delicate mocha dacquoise for dessert, how can you turn it down? She pours coffee.

Next thing you know, you are sipping an after-dinner drink — bourbon, of course. You sheepishly glance at your watch. Has it really been three hours since that first drink?

The historic Brown Hotel is THE place to party on Derby Day, but you don't have to wait for the first weekend in May to sip a refreshing mint julep in the genteel opulence of the Lobby Bar.

This cocktail made with bourbon and homemade, mint-infused simple syrup is almost as much a part of the Kentucky Derby as big hats and lightening-fast thoroughbreds.

Derby Day or not, the Brown is the sort of place where a gentleman sipping bourbon in a seersucker suit and bow tie would not be out of place.

Maker's Mark is the bourbon of choice for mint juleps at the Brown, but if you prefer another bourbon, the shelves are stocked with almost every imaginable label.

Bartender Ron Bacigalupi points out that the mint julep is just one of several drinks the Brown does like nobody else.

He says when it comes to trendy bourbon cocktails, what's old is new again.

"The connoisseurs are asking for traditional bourbon cocktails like Manhattans or Old Fashioneds with muddled fruit."

Other customers don't want fruit or anything else interfering with their bourbon.

"Those on the Bourbon Trail want to experience the bourbon's distinct flavor profile, especially with the single-barrel and small-batch bourbons," Bacigalupi says. "Bourbon drinkers have become more aware of flavors like cinnamon, nutmeg, vanilla and wood. Sometimes a splash of water will bring out those flavors even more."

Another not-to-be-missed historic hotel is the Seelbach, a familiar destination to F. Scott Fitzgerald fans. The author of "The Great Gatsby" stayed at the Seelbach and was so inspired by its elegance that it became the setting for Tom and Daisy Buchanan's wedding.

The Old Seelbach Bar feels like an old-school gentlemen's club, definitely not feminine enough for Daisy, yet she might have enjoyed sipping the signature Seelbach cocktail in the luxurious lobby.

The cocktail was first crafted in 1917 by a harried barkeep who mistakenly poured Champagne into an Old Fashioned. The fortuitous concoction was a hit and was served regularly until Prohibition, when the recipe was lost for more than six decades. The fizzy cocktail was rediscovered in 1995, and it's appreciated today as much as it was during the Jazz Age.

Bourbon was once a spirit enjoyed mostly by Southerners. Like so many good things about the South, it took the rest of the world a while to discover it, but now Southern hospitality in a glass can be had almost anywhere.

But only in Louisville can you drink fine bourbon so close to its source and have it served by some of the most knowledgeable bartenders (let's call them bourbonoligists) you are ever likely to meet.

To pick up the Urban Bourbon Trail passport, stop by the Louisville Visitors Center downtown at Fourth and Jefferson or any of the participating stops. Just ask your server. An electronic version can be downloaded by using the iphone or Android apps.

When you have acquired six stamps -- you need not get them all in one day, and food and non-bourbon count -- stop by the Visitors Center to get your T-shirt and certificate or have them sent through the mail by following the directions in the back of your passport.